It has become a matter of serious public concern recently that one source of air pollution is the exhaust gas of automobiles, which contains a large amount of harmful unburned gas such as carbon monoxide and hydrocarbon.
It is known that, when these harmful gases are mixed with a proper quantity of secondary air before exhausting into the atmosphere, combustion again takes place and almost all the unburned gases are burned. Devices for such purposes have been developed which include a port liner, of which the exhaust port portion in the cylinder head is arranged to retain heat, an exhaust manifold of large capacity, a manifold reactor, and an afterburner.
These devices, however, have various disadvantages in that the adiabatic effect is small and performance insufficient for cleaning exhaust gas, their durability is inferior over an extended period of use at high temperatures, the devices are complicated, or their manufacture is not economical, i.e. low volume production and/or high cost, because the devices are made mainly of metallic material.
An attempt has been made to overcome these difficulties by using a ceramic material having a lower thermal conductivity than metallic material and which is more durable at high temperatures in combination with metallic material. In this device only the outer surface of a hollow ceramic core is made porous, and molten metal is poured to solidify on the outer side of the core to form a composite pipe of metal-ceramics.
In making this composite pipe attention must be paid to prevent either the metal or ceramic side from failing when solidifying or cooling; accordingly, only the outer surface of the ceramic is made porous, the ceramic is set as necessary and a casting mold is heated as a whole to pour molten metal so that the metal may be fully impregnated into the porous cavities on the ceramic surface.
As a result, the compressive force produced when the metal is solidified and cooled is released due to a yielding phenomenon occurring locally in the mechanical joint portion between the ceramic permeating into metal and the metal. The result is a composite structure having a ceramic side and a metal side at the juncture of which there is no apparent failure.
Nonetheless, the composite material formed according to this method is in a yielding state where minute cracks exist potentially in the mechanical joint portion of the ceramic and metal, although there exists no apparent crack. Consequently, when the composite is used in a system where it is subject to severe vibration, as in an automobile engine, or the like, cracking between the ceramic and metal develops rapidly. It is proved by experience that the durability of this construction to mechanical vibration is markedly inferior.
It is also known that ceramics which are porous only on the outer surface are produced by complicated processes and, as a result, it is difficult to manufacture complicated parts of exhaust systems of engines or relatively thin-walled parts.